Interlayer Research Articles

Optimizing GaN () hetero‐epitaxial templates grown on () sapphire

AbstractThe hetero‐epitaxy of () GaN on () sapphire was optimized in metal–organic vapor phase epitaxy. Best results were obtained from an AlN nucleation followed by AlN and AlGaN layers, and inserting low‐temperature AlN interlayers (ILs) as well as a SiNx IL. X‐ray diffraction (XRD) of scans of the symmetric () reflection yielded an FWHM along [] and along [] together with a m rms roughness below 10 nm as determined by atomic force microscopy. The lowest threading dislocation density achieved was cm while the basal plane stacking fault density was in the lower cm range as determined by transmission electron microscopy. The suppression of the unwanted () phase was lower than 1 in 10,000 as judged from XRD.

Influence of Chemical Composition and Structure in Silicon Dielectric Materials on Passivation of Thin Crystalline Silicon on Glass.

In this study, various silicon dielectric films, namely, a-SiOx:H, a-SiNx:H, and a-SiOxNy:H, grown by plasma enhanced chemical vapor deposition (PECVD) were evaluated for use as interlayers (ILs) between crystalline silicon and glass. Chemical bonding analysis using Fourier transform infrared spectroscopy showed that high values of oxidant gases (CO2 and/or N2), added to SiH4 during PECVD, reduced the Si-H and N-H bond density in the silicon dielectrics. Various three layer stacks combining the silicon dielectric materials were designed to minimize optical losses between silicon and glass in rear side contacted heterojunction pn test cells. The PECVD grown silicon dielectrics retained their functionality despite being subjected to harsh subsequent processing such as crystallization of the silicon at 1414 °C or above. High values of short circuit current density (Jsc; without additional hydrogen passivation) required a high density of Si-H bonds and for the nitrogen containing films, additionally, a high N-H bond density. Concurrently high values of both Jsc and open circuit voltage Voc were only observed when [Si-H] was equal to or exceeded [N-H]. Generally, Voc correlated with a high density of [Si-H] bonds in the silicon dielectric; otherwise, additional hydrogen passivation using an active plasma process was required. The highest Voc ∼ 560 mV, for a silicon acceptor concentration of about 10(16) cm(-3), was observed for stacks where an a-SiOxNy:H film was adjacent to the silicon. Regardless of the cell absorber thickness, field effect passivation of the buried silicon surface by the silicon dielectric was mandatory for efficient collection of carriers generated from short wavelength light (in the vicinity of the glass-Si interface). However, additional hydrogen passivation was obligatory for an increased diffusion length of the photogenerated carriers and thus Jsc in solar cells with thicker absorbers.

Improved Structural and Electric Characterstics of Al/ALD-HfO2/Ge MOS Capacitor by Germanium Dioxide and Germanium Oxynitride as Interfacial Layer

In this work HfO2 thin films has been deposited on Ge substrate by ALD with thickness of 9.77 nm. Thickness evaluation of film was done by Spectroscopic Ellipsometer (SE) and Filmetrics Reflectometer whereas chemical compositions and the states of the elements of the films were investigated by X-ray Photoelectron Spectroscopy (XPS). High-frequency Capacitance-Voltage (C-V), Conductance-Voltage (G-V) techniques and Current-Voltage (I-V) measurement were used to obtain the electrical properties of the high-k MOS capacitor. Different Ge surface cleaning methods preceding high-k ALD have been used for reducing oxide traps. Rapid Thermal Oxidation (RTO) and Rapid Thermal Nitridation (RTN) have been used to form the gate interlayer (IL) of GeO2 and GeON for the gate stack of P-MOS capacitor. By assimilating a thin GeON interlayer, excellent MOS capacitors with very small capacitance–voltage hysteresis and low gate leakage have been demonstrated. Physical and Electrical characterization investigates the quality of the oxynitride interlayer and low interface trap density (Dit ≈ 0.63×1012eV-1cm-2) in case of GeON/Ge which is improved as compared to the GeO2/Ge interface (Dit ≈ 2.53×1013 eV-1cm-2). Further electrical characterizations of HfO2 gate dielectric MOS capacitors deposited by the ALD using RTO & RTN as IL were investigated for the measurement of EOT and Qeff. The calculated EOT and Qeff values for GeON/Ge are 3.23 nm and 16.92×1012 cm-2 respectively which is better in comparison with GeO2 and as deposited samples.

Improved Structural and Electric Characterstics of Al/ALD-HfO2/Ge MOS Capacitor by Germanium Dioxide and Germanium Oxynitride as Interfacial Layer

A 9.77 nm HfO2 thin film has been deposited on Ge substrate by ALD method. Rapid Thermal Oxidation (RTO) and Rapid Thermal Nitridation (RTN) have been used to form the gate interlayers (ILs) of GeO2 and GeON between them. High-frequency Capacitance-Voltage (C-V), Conductance-Voltage (G-V) techniques and Current-Voltage (I-V) measurement were used to obtain the electrical properties of the high-k MOS capacitor. By assimilating a thin GeON interlayer, excellent MOS capacitors with very small capacitance–voltage hysteresis and low gate leakage have been demonstrated. The results suggest that the physical and electrical properties (K, Dit, Qeff, etc) improved by rapid thermal annealing (RTA) in nitrogen ambient, which improves the interface properties of HfO2/Si due to the densification of HfO2 thin films. We found that the GeON/Ge gate stack exhibit a reduced equivalent oxide thickness (3.23 nm), interfacial trap density, effective oxide charges (Qeff), C–V hysteresis and leakage current.

Performance improvements of AlGaN/GaN HEMTs by strain modification and unintentional carbon incorporation

An advanced AlGaN/GaN HEMT structure, grown on a sapphire substrate by MOCVD utilizing a high temperature (HT) AlN interlayer (IL) and a multilayer high-low-high temperature (HLH) AlN buffer layer, demonstrates a superior performance both in breakdown voltage (>200 V) and maximum drain current (IDSS = 667 mA/mm). The HT AlN IL produces an additional compressive strain into the above GaN layer. Accordingly, an AlGaN barrier, grown on the more compressive GaN, introduces less tensile strain leading to an improvement in surface morphology (RMS = 0.19 nm in 2 × 2 μm2), a remarkable increase in 2DEG mobility by 46% (μ s = 1900 cm2/Vs) and a decrease in densities of defects acting as paths for the leakage current through the AlGaN barrier. A high semi-insulating buffer is achieved by eliminating leakage paths both through the buffer layer and the buffer-substrate interfacial layer. These result from an increase in unintentional carbon introduced by AlN layers, especially by a low temperature AlN layer; which are grown under low pressure (50 Torr). Lastly, the decrease in AlGaN barrier tensile strain and low leakage current in the advanced HEMTs structure using an HT AlN IL and an HLH AlN buffer are promising for an improvement in AlGaN/GaN HEMTs’ reliability.

On the increased efficiency in InGaN-based multiple quantum wells emitting at 530–590 nm with AlGaN interlayers

InGaN/AlGaN/GaN-based multiple quantum wells (MQWs) with AlGaN interlayers (ILs) are investigated, specifically to examine the fundamental mechanisms behind their increased radiative efficiency at wavelengths of 530–590nm. The AlzGa1−zN (z~0.38) IL is ~1–2nm thick, and is grown after and at the same growth temperature as the ~3nm thick InGaN quantum well (QW). This is followed by an increase in temperature for the growth of a ~10nm thick GaN barrier layer. The insertion of the AlGaN IL within the MQW provides various benefits. First, the AlGaN IL allows for growth of the InxGa1−xN QW well below typical growth temperatures to achieve higher x (up to~0.25). Second, annealing the IL capped QW prior to the GaN barrier growth improves the AlGaN IL smoothness as determined by atomic force microscopy, improves the InGaN/AlGaN/GaN interface quality as determined from scanning transmission electron microscope images and x-ray diffraction, and increases the radiative efficiency by reducing non-radiative defects as determined by time-resolved photoluminescence measurements. Finally, the AlGaN IL increases the spontaneous and piezoelectric polarization induced electric fields acting on the InGaN QW, providing an additional red-shift to the emission wavelength as determined by Schrodinger-Poisson modeling and fitting to the experimental data. The relative impact of increased indium concentration and polarization fields on the radiative efficiency of MQWs with AlGaN ILs is explored along with implications to conventional longer wavelength emitters.

Barrier Strain and Carbon Incorporation‐Engineered Performance Improvements for AlGaN/GaN High Electron Mobility Transistors**

AbstractThe improvements in electrical characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) grown using metal‐organic (MO)CVD by engineering structure, barrier strain, and unintentional carbon incorporation, are demonstrated in this work. Both normal HEMT structure (with a high temperature (HT) AlN buffer) and advanced HEMT structure (with a high‐low‐high temperature (HLHT) AlN buffer, and a HT AlN interlayer (IL)) present a breakdown voltage higher than 200 V, while a much smaller breakdown voltage of 17 V is measured on the conventional structure using a low‐temperature GaN buffer. The HT AlN IL inserted in the middle of the conventional HEMT structure introduces a reduction in the tension of the AlGaN barrier, which results in an improvement of the surface morphology (0.46 nm). As a consequence, the two‐dimensional electron gas (2DEG) mobility increases by remarkable 46% (1900 cm2 V−1 s−1). The HLHT AlN buffer, substituting for the HT AlN buffer, leads to the enhancement of GaN crystalline quality, which contributes to the performance improvement for HEMTs. The advanced HEMT, using both an AlN IL and an HLHT AlN buffer, produces increases in the DC maximum drain current by 35.5% (∼680 A mm−1), and in the transconductance by 15% (114 mS mm−1) in comparison with the normal HEMT with an AlN buffer. The very low leakage current in the advanced HEMTs is caused by optimizing the design of the buffer and modifying growth parameters. Lastly, the reduction of AlGaN barrier tensile strain by inserting the HT AlN IL is promising for an improvement in AlGaN/GaN HEMT reliability.

Wireless Video: An Interlayer Error-Protection-Aided Multilayer Approach

Video clips captured from realworld scenes exhibit intraframe correlation among their pixels. This correlation can be removed by applying video compression to reduce the required the storage space, transmission bandwidth, bitrate, and power. Layered video coding separates the video sequence into partitions having unequal importance, hence allowing the decoder to progressively refine the reconstructed video quality, when an increased bandwidth is available. On the other hand, compressed video signals are sensitive to channel errors. Therefore, forward error correction (FEC) must be applied when communicating over hostile wireless channels. In addition, based on the fact that the different layers have unequal importance, different-rate FEC codes may be applied to the different layers, leading to unequal error protection (UEP). We propose an interlayer (IL) FEC coding technique combined with UEP, where the lower-importance layers are used for protecting the higher-importance layers in the data-partitioned mode of H.264/advanced video coding (AVC). Explicitly, our simulation results show that the IL coded system outperforms the traditional UEP system by providing a better video quality for transmission over a wireless channel having Eb /N0 of 0 dB, when using our multifunctional multiple-input, multiple-output (MIMO) array.

Epitaxial evolution on buried cracks in a strain-controlled AlN/GaN superlattice interlayer between AlGaN/GaN multiple quantum wells and a GaN template

Epitaxial evolution of buried cracks in a strain-controlled AlN/GaN superlattice interlayer (IL) grown on GaN template, resulting in crack-free AlGaN/GaN multiple quantum wells (MQW), was investigated. The processes of filling the buried cracks include crack formation in the IL, coalescence from both side walls of the crack, build-up of an MQW-layer hump above the cracks, lateral expansion and merging with the surrounding MQW, and two-dimensional step flow growth. It was confirmed that the filling content in the buried cracks is pure GaN, originating from the deposition of the GaN thin layer directly after the IL. Migration of Ga adatoms into the cracks plays a key role in the filling the buried cracks.

Novel Ni silicide formed with a Ni/Er/Ni/TiN structure for thermal stable and low contact resistance source/drain in MOSFETs

In this paper, Ni silicide formed with a Ni/Er/Ni/TiN structure is proposed, and the thermal stability and electrical properties of the proposed Ni silicide with an Er interlayer in different positions are studied for application to source/drain of MOSFETs. Ni silicide with Ni/Er/Ni/TiN structures showed not only improved thermal stability but also lower sheet resistance than an Er/Ni/TiN structure. Moreover, proposed Ni/Er/Ni/TiN (2/2/11/10 nm) structure exhibited great barrier height and low ideality factor similar to the Er/Ni/TiN structure which could reduce the contact resistance between silicide and doped source/drain region. Therefore, proposed silicide using Ni/Er/Ni/TiN (2/2/11/10 nm) structure is promising for MOSFET source/drain due to better thermal stability, low sheet resistance, and excellent electrical properties.

Understanding the role of ultra-thin polymeric interlayers in improving efficiency of polymer light emitting diodes

Insertion of ultra-thin polymeric interlayers (ILs) between the poly(3,4-ethylenedioxythiophene):polystyrene sulphonate hole injection and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) light emission layers of polymer light emitting diodes (PLEDs) can significantly increase their efficiency. In this paper, we investigate experimentally a broad range of probable causes of this enhancement with an eye to determining which IL parameters have the most significant effects. The importance of hole injection and electron blocking was studied through varying the IL material (and consequently its electronic energy levels) for both PLED and hole-only diode structures. The role of IL conductivity was examined by introducing a varying level of charge-transfer doping through blending the IL materials with a strong electron-accepting small molecule in concentrations from 1% to 7% by weight. Depositing ILs with thicknesses below the exciton diffusion length of ∼15 nm allowed the role of the IL as a physical barrier to exciton quenching to be probed. IL containing PLEDs was also fabricated with Lumation Green Series 1300 (LG 1300) light emission layers. On the other hand, the PLEDs were modeled using a 3D multi-particle Kinetic Monte Carlo simulation coupled with an optical model describing how light is extracted from the PLED. The model describes charge carrier transport and interactions between electrons, holes, singlets, and triplets, with the current density, luminance, and recombination zone (RZ) locations calculated for each PLED. The model shows F8BT PLEDs have a narrow charge RZ adjacent to the anode, while LG 1300 PLEDs have a wide charge RZ that is evenly distributed across the light emitting layer. Varying the light emitting layer from F8BT to Lumation Green Series 1300, we therefore experimentally examine the dependence of the IL function, specifically in regard to anode-side exciton quenching, on the location of the RZ. We found an exponential dependence of F8BT PLED luminance on the difference, δ, in the highest occupied to lowest unoccupied molecular orbital energy gap between the light emitting polymer and a semiconducting polymeric IL, with δ consequently the most important parameter determining efficiency. Understanding the exponential effect that wider energy gap IL materials have on exciton quenching may allow δ to be used to better guide PLED structure design.

Intra- and Interlayer Electron-Phonon Interactions in 12/12C and 12/13C BiLayer Graphene

This review focuses on intra- and interlayer (IL) electron-phonon interactions and phonon self-energy renormalizations in twisted and AB-stacked bilayer graphene (2LG) composed either only of 12C or a mixing of 12C and 13C isotopes. A simple way to imagine a 2LG is by placing one monolayer graphene (1LG) on top of another 1LG. The orientation of one of the layers with relation to the other may originate a twisted 2LG system (known as turbostratic) as well as a AB-stacked system, also known as Bernal stacking. By rotating the layers of a 2LG one can departure from a fully misoriented system to achieve the AB-stacked configuration and their IL interactions can be dramatically different being close to zero in a fully misoriented system and maximum in an AB-stacked system. Interlayer interactions are expected to slightly perturb the intralayer phonons and they also govern the low-energy electronic and vibrational properties, which are of primary importance to phenomena such as transport, infrared (IR) optics and telecommunication bands in the IR range. Therefore, a comprehensive discussion combining intra- and interlayer phenomena is necessary and addressed throughout the text.

Role of an interlayer at a TiN/Ge contact to alleviate the intrinsic Fermi-level pinning position toward the conduction band edge

TiN/Ge contacts, prepared by direct sputter deposition from a TiN target, can alleviate the intrinsic Fermi-level pinning (FLP) position toward the conduction band edge. This work focuses on studying the origin of the FLP alleviation. Investigations on both the electrical properties and interfacial structures of TiN/Ge contacts showed that an amorphous interlayer (IL) containing nitrogen played an important role in the alleviation. For comparison, the properties of Ti/Ge contacts were also studied. Based on these results, the IL structure that induced the FLP alleviation was clearly shown and a model was proposed to explain the FLP alleviation.

Novel silicon surface passivation by Al2O3/ZnO/Al2O3 films deposited by thermal atomic layer deposition

In this paper, a novel Al2O3/ZnO/Al2O3 stack is proposed as the silicon passivation layer for c-Si solar cell application. Recently, the Al2O3 film has been proved to be effective for passivating the p-type c-Si surface by forming the negative fixed oxide charge. It is confirmed by this experiment that the amount of negative fixed oxide charge can be controlled by inserting a ZnO interlayer (IL), which is explained by acceptor-like defect (VZn, Oi, and OZn) formation determined by the room-temperature photoluminescence (RTPL) analysis. The effect of ZnO IL is investigated using Al2O3 bottom layers of various thicknesses by electrical and physical analyses. The effective lifetime measurement shows that the electronic recombination losses at the silicon surface are reduced effectively by optimizing the Al2O3/ZnO/Al2O3 stack.

Polyethylene Imine as an Ideal Interlayer for Highly Efficient Inverted Polymer Light‐Emitting Diodes

Electron‐injecting interlayers (ILs) which are stable in air, inject electrons efficiently, block holes, and block quenching of excitons, are very important to realize efficient inverted polymer light‐emitting diodes (IPLEDs). Two air‐stable polymer electron‐injecting interlayers (ILs), branched polyethyleneimine (PEI) and polyethyleneimine ethoxylated (PEIE) for use in IPLEDs are introduced, and the roles of the ILs in IPLEDs comparing these with a conventional Cs2CO3 IL are elucidated. These polymer ILs can reduce the electron injection barrier between ZnO and emitting layer by decreasing the work function (WF) of underlying ZnO, thereby effectively facilitating electron injection into the emitting layer. WF of ZnO covered by PEI is found to be lower than that covered by PEIE due to higher [N+]/[C] ratio of PEI. Furthermore, they can block the quenching of excitons and increase the luminous efficiency of devices. Thus, IPLEDs with PEI IL of optimum thickness (8 nm) show current efficiency (13.5 cd A–1), which is dramatically higher than that of IPLEDs with a Cs2CO3 IL (8 cd A‐1).

GaN-based high-electron-mobility transistor structures with homogeneous lattice-matched InAlN barriers grown by plasma-assisted molecular beam epitaxy

Metal-polar In0.17Al0.83N barriers, lattice-matched to GaN, were grown under N-rich conditions by plasma-assisted molecular beam epitaxy. The compositional homogeneity of these barriers was confirmed by plan-view high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography. Metal-polar In0.17Al0.83N/(GaN)/(AlN)/GaN structures were grown with a range of AlN and GaN interlayer (IL) thicknesses to determine the optimal structure for achieving a low two-dimensional electron gas (2DEG) sheet resistance. It was determined that the presence of a GaN IL was necessary to yield a 2DEG sheet density above 2 × 1013 cm−2. By including AlN and GaN ILs with thicknesses of 3 nm and 2 nm, respectively, a metal-polar In0.17Al0.83N/GaN/AlN/GaN structure regrown on a GaN-on-sapphire template yielded a room temperature (RT) 2DEG sheet resistance of 163 Ω/□. This structure had a threading dislocation density (TDD) of ∼5 × 108 cm−2. Through regrowth on a free-standing GaN template with low TDD (∼5 × 107 cm−2), an optimized metal-polar In0.17Al0.83N/GaN/AlN/GaN structure achieved a RT 2DEG sheet resistance of 145 Ω/□ and mobility of 1822 cm2 V−1 s−1. High-electron-mobility transistors with output current densities above 1 A mm−1 were also demonstrated on the low-TDD structure.

Strain states in GaN films grown on Si(111) and Si(110) substrates using a thin AlN/GaN superlattice interlayer

AbstractWe report the characterization results of the strain states in GaN films grown both on Si(111) and Si(110) substrates by applying a thin AlN/GaN superlattice structure (SLs) as an interlayer (IL). The strain states in the GaN films grown on Si(111) and Si(110) substrates are compared. It is demonstrated that the modulation effect by the SL ILs on the strain state in the GaN film is universal although different behaviours on the Si(111) and Si(110) substrate are observed due to the different lattice symmetry relationship between the substrate and the epilayer. By using the technique, we succeeded in growing thick continuous GaN films (> 3 µm), which are necessary for the high‐power, high‐voltage device applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A Review on the Use of Stress Absorbing Membrane Inter Layers (SAMIs) as Remedy for Reflective Cracks in Pavement

A Review on the Use of Stress Absorbing Membrane Inter Layers (SAMIs) as Remedy for Reflective Cracks in Pavement

Modulation of electron barriers between TiNx and oxide insulators (SiO2, Al2O3) using Ti interlayer

Precise evaluation of the effective work function and the built‐in voltage in metal–oxide–metal (MOS) or metal–insulator–metal (MIM) stacks is crucial for attaining the functionality of various oxide‐based electron devices. We studied the impact of a low work function (WF) metal interlayer (IL), i.e. Ti, on the electron barrier height between the metal Fermi level and the conduction band of the oxide insulator. The experiments were performed on structures comprised of a layer of thermal SiO2 or an atomic layer deposited Al2O3 on heavily doped p‐ or n‐Si substrates. In order to evaluate the impact of the Ti IL, 3, 6 or 10 nm thick Ti films have been deposited prior to TiNx sputtering. Spectral curves of electron photoemission from the metal layer into the oxide were analyzed in order to determine the energy barrier for electrons between the Fermi level of the metal and the conduction band of the insulator (Φe). As expected, the insertion of the low WF metal shifts the barrier to lower energies, depending on the IL thickness. However, while a 3‐nm thick Ti IL is sufficient to lower the barrier by 0.5 eV in the SiO2‐based stack, it has no measurable impact on the barrier in the Al2O3/Ti/TiNx stack. For the latter structure, both Ti and the Al2O3 thicknesses influence the Φe lowering trend, suggesting that oxygen scavenging from Al2O3 by Ti and nitridation of the Ti IL by subsequent TiN deposition may affect the band alignment between the metal and the oxide.

Germanium oxynitride gate interlayer dielectric formed on Ge(100) using decoupled plasma nitridation

Germanium Oxynitride (GeON) gate interlayer (IL) dielectric formed using decoupled plasma nitridation (DPN) technique is compared with GeO2 and thermally nitrided GeON ILs for Ge gate stack applications using n-channel capacitors and transistors. Lower nitrogen concentration and roughness at the GeON/Ge interface lead to lower midgap interface trap density (Dit) and 1.5× higher electron mobility for the DPN versus thermally nitrided GeON IL. DPN GeON IL also exhibits enhanced thermal stability till 575 °C at the expense of a small degradation in Dit versus GeO2 IL, making it a more viable gate IL dielectric on Ge channels.